The Light Emitting Diode (LED) is revolutionizing the lighting world. Its high and still increasing luminous efficacy, its long lifetime, its small form factor, its low weight, its ruggedness, its ease of manufacturing, etc., form driving forces for the lighting industry to move from the incandescent lamp to the LED lamp. However, the LED's non-linear (near exponential) I-V curve, its low operation voltage (a few volt) and its fast response of the current-to-light-output transfer create a number of problems when the lamp is to be supplied from mains voltage (e.g. 115 V AC or 230 V AC), including light output flicker and voltage misalignment.
Ideally the LEDs have to be supplied with a direct current as the transfer from current to light output is nearly linear. A solution is to apply an AC-DC converter to generate a DC supply for the LEDs from the mains input voltage. Such a converter can be controlled in such a way that the output current is set to a prescribed level. Preferably the prescribed level can be changed by actions undertaken by dimmers that are connected e.g. in series with the mains Live connection and the lamp. In order to realize a power-efficient solution, a common approach is to use a switched-mode power supply, built with (semiconductor) switches and inductive energy-storage components such as inductors or transformers. Especially the inductive components add to the cost and physical volume of the system.
The Tapped Linear Driver (TLD) concept as e.g. described in U.S. Pat. No. 6,989,807 B2, U.S. Pat. No. 7,081,722 B1 or U.S. 2008/0094000 A1, allows significant cost reduction for future mains-compatible drivers for LED lighting systems. Due to its small form factor it is suited for integrated LED light sources such as LED retrofit bulbs and spots, but also for down-light modules. The Tapped Linear Driver concept is low cost as it avoids the use of an inductive switched-mode power supply. It is based on applying high-voltage LEDs. Essentially a high-voltage voltage LED is a multi-junction LED, a string of series-connected LEDs, such that the high-voltage LED forward-biased voltage is several tens of volts when the LEDs emit light. In a Tapped Linear Driver several high-voltage LEDs are connected in series and the nodes at which the high-voltage LEDs are interconnected form the taps. Depending on the instantaneous value of the (rectified) mains voltage more or less high-voltage LEDs are supplied with current.
The current is supplied by a linear (non-switching) current source, in its simplest form a resistor. According to U.S. Pat. Nos. 6,989,807 B2 and 7,081,722 B1 the currents are supplied by parallel current sources, which offer the option to allocate different current values to the various current sources. According to U.S. 2008/0094000 A1 a single (constant) current source circuit is applied in series with the LED string while again multiple switches addressed by a logic circuit bypass the LEDs.
All of these TLD systems have one common disadvantage: if the (rectified) mains voltage falls below the voltage of the “shortest” high-voltage LED string, there is no light output. This happens around the zero crossing of the mains AC voltage. A solution proposed e.g. in WO 2010/027254 A1 is to insert a so-called fill-in capacitor. The fill-in capacitor is charged to nearly the peak value of the rectified mains voltage (+325V when the mains RMS voltage is 230V AC) and it supplies the energy to the LEDs for the time that the rectified mains voltage is “too low”, in practice this is below a certain threshold value. During that time the diodes in the mains bridge rectifier are reverse biased and the fill-in capacitor is discharged by the LEDs from nearly +325V to e.g. +280V depending on various design parameters and choices.
Another disadvantage is the reduced efficiency. The linear current source will dissipate a certain amount of energy into heat, where the amount depends on the mismatch between momentary (rectified) mains and the available LED string voltage taps. Since the number of taps is normally limited (in order to avoid very high complexity and part count for the driver circuit) this voltage step and hence the mismatch and losses can be quite significant, resulting in driver efficiencies of approximately 80%.
When such a circuit is used with a dimmer in the mains supply branch a new problem arises. During the time that the fill-in capacitor is used to supply the energy, the current drawn from mains is substantially zero as the diodes in the mains bridge rectifier are reverse-biased, and this may lead to improper operation of the internal timing circuit of the dimmers, e.g. two-wire dimmers such as TRIAC dimmers. Such a dimmer is designed to operate with an incandescent lamp, which provides a conductive path all the time since the lamp itself is a resistor. Once a (non-dimmable) LED lamp is connected to the dimmer, this conductive path does not always exist. In most cases the lamp current stops flowing before the zero-crossing of the mains voltage. This results in no or incorrect activation of the power switch in the dimmer and leads to no or unstable light output (light flicker), which of course is unwanted.
In case of the power switch being a TRIAC, light flicker will also happen if the current drawn by the lamp is below the holding current of the TRIAC. In this situation, the dimmer stops conducting and the timing circuit may restart and trigger the TRIAC once again. This sequence may repeat. This is normally referred to as “multi-triggering” and also is unwanted.
The addition of the fill-in capacitor helps to maintain light output around the zero crossing of the mains voltage (flicker reduction), but creates problems with dimmers such as multi-triggering as its discharging current does not pass through the mains branches.
In general, in Solid State Lighting (SSL), i.e. in general illumination, the relative cost contribution of the driver is expected to increase due to the decrease in LED cost. So, in order to reduce the cost of the total system at a given performance level, also the driver has to become cheaper (in particular simpler and smaller) and/or more efficient. A closer linkage between LED and driver will still enable high performance. Besides the cost, for some countries certain mains regulations have to be fulfilled, such as low harmonic distortion and/or high power factor.